1. Field of the Invention
The present invention relates generally to depth gauges for scuba divers and more specifically to an improved oil-filled depth gauge of the type having a maximum depth indicator, the improvement residing in a modification for reducing the likelihood of an inaccuracy in the maximum depth indicator reading.
2. Prior Art
One of the most important instruments carried by a scuba diver is his depth gauge, which indicates how far he is below the surface of the water. Knowledge of the depth adds to the enjoyment of the dive as well as supplying depth information which dictates precautions and procedures for safe and efficient diving during both the normal and the extreme. The human body has no reliable means of discriminating depth accurately even though it is quite sensitive to this change in depth and water pressure. There are many factors that should be considered when selecting the depth gauge, but probably the most important is accuracy. The accuracy of the depth gauge depends upon the following three elements: Repeatability of the gauge movement; Accuracy of the transmission of the surrounding pressure to the gauge mechanism; and accuracy of the calibration. Repeatability can be defined as the ability of the depth gauge mechanism to consistently produce the same readings. Such overall consistency depends upon the sensitivity of the pressure sensing device and the accuracy of the gear drive mechanism. Superior depth gauge devices use oil-filled capsule depth gauges having a Bourdon spring tube coupled to an indicator needle with a rack and pinion gear drive. The accuracy of the depth gauge is affected by the internal friction or hysteresis of the mechanism. Hysteresis can be minimized by providing an oil suspension specifically formulated to have the least amount of friction and the lowest viscosity change through the normal temperature range encountered in diving. In addition, the ambient pressure transfer diaphragm must be made from a material which maintains the accuracy of the finished gauge to within a small tolerance of the original mechanism in water temperatures ranging from about 32 degrees Fahrenheit to 90 degrees Fahrenheit. Normal calibration accuracies for Bourdon tubes in standard depth gauges is plus or minus 2% of the full scale, regardless of location of the needle on the dial face. In order for manufacturers to be sure that the mechanism stays within this accuracy, they calibrate the gauge to read most accurately at the mid-point of the dial. Thus, a two hundred foot gauge, for example, can vary by plus or minus four feet anywhere on the dial. At twenty feet, the gauge could read sixteen to twenty four feet and still be acceptable by the plus or minus 2% standard. A critical parameter readout provided by such depth gauges is the maximum depth to which a diver has a descended during his dive. This parameter is of critical importance because it determines how much time the diver must reserve for ascending to the surface without running the risk of nitrogen sickness or requiring decompression to overcome such an affliction. Typically, the maximum depth indicator is a simple mechanical dial needle which is pushed by the instantaneous depth gauge dial needle but which can be manually returned to zero at the end of the dive. Accordingly, the maximum depth gauge dial reads the greatest depth to which the diver has descended during his dive.
One of the most precise scuba diving depth gauges is filled with oil, which makes it more sensitive to pressure variations and less sensitive to temperature variations.
Unfortunately, there is a potential disadvantage associated with oil-filled depth gauges, particularly in regard to the potential influence of the oil on the maximum depth readout. This disadvantage is likely to occur whenever the gauge is inadvertently jarred, such as when a wrist worn gauge is inadvertently bumped against a rock or other surface. Such jarring may result in a sudden relative acceleration difference between the body of the gauge, and the oil contained therein. When this occurs, it is likely that this relative difference in acceleration between the oil and the body of the gauge will cause motion of the maximum depth gauge indicator and obviously such motion destroys the accuracy of the maximum depth reading. Inaccuracy in the maximum depth gauge reading, can as a minimum be a great inconvenience and even potentially hazardous. If the depth gauge reading is moved to (erroneously indicate) a greater maximum depth, it may cause the diver to abort his dive earlier than necessary in order to rise to the surface at an earlier time. On the other hand, if the movement of the maximum depth gauge reading reduces the reading which results in an erroneous shallower maximum depth indication, which results in a false shallower depth reading, the diver may stay below the surface longer than he should to avoid the requirements of decompression and thus incur severe physical injury. Accordingly, there has been a long felt need for an accurate scuba diving depth gauge which is less likely to be affected by violent shock so that the maximum depth gauge indication is more accurate, irrespective of such violent shock.